Multidirectional Human-in-the-Loop Balance Robotic System

Abstract

We present the design and performance of a novel multidirectional balance robotic system. This robot adds mediolateral standing balance control to the functionality of previous systems. To evaluate the performance of the system, we quantified its motion-tracking capability by applying target trajectories related to perturbed and unperturbed quiet standing balance. We observed minimal delays (9.4–13.9 ms) and near-unity gain up to 4 Hz when tracking multi-sine trajectories and small errors when tracking natural balance trajectories (≤0.05 mm, corresponding to ≤0.009 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$^\circ $</tex-math></inline-formula> ), which were all below human perceptual thresholds reported for standing balance (150 ms and 0.17 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$^\circ $</tex-math></inline-formula> at 0.06 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$^\circ $</tex-math></inline-formula> /s). Next, we evaluated the human-in-the-loop real-time robot performance when participants (N = 6) maintained their upright balance in the mediolateral direction while firmly secured to the robot. The results revealed small errors between the predicted and robotic motion (<0.4 mm, corresponding to <0.03 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$^\circ $</tex-math></inline-formula> ) as well as between the robotic and measured human motion (<0.7 mm, corresponding to <0.05 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$^\circ $</tex-math></inline-formula> ) in real-time applications. The sub-15ms delays, combined with submillimeter movement errors and relative robot-human movements, enable realistic multidirectional simulations of human balance. These unique robotic features open up new research opportunities for exploring the sensorimotor principles and biomechanical interactions underlying the multidirectional control of balance, and may ultimately be used to assess and rehabilitate standing balance deficits in older adults and clinical populations.